Reverse time migration (RTM) is a well-established seismic imaging technology especially designed for areas of complex geology. RTM uses the two-way wave equation to improve the handling of velocity complexity and to image the steep dips by utilizing reflections, transmissions, diffractions and prism waves. Previous methods have discussed how RTM can be adapted to generate velocity and impedance perturbations from an analysis of output angle gathers. This adaptation of RTM was applied in the context of the acoustic approximation and subsequently ended up with an acoustic version of Aki, K., and P. Richards, “Quantitative Seismology: Theory and Methods”, W.H. Freeman and Co. (1980) approximation to the P-wave to P-wave (PP) Zoeppritz equation.
In general, accurate velocity, impedance and/or elastic model perturbations are extremely valuable in the seismic imaging problem, whether for directly updating the underlying velocity, impedance, density, anisotropy or elastic parameters in an Earth model, or for use as an interpretation tool. However, the previous uses of RTM to generate angle gathers was applied to an acoustic version of the Zoeppritz equation. The use of the acoustic wave equation limits the application of these previous uses to acoustic or pressure waves. However, real seismic data includes both pressure and shear wavefields as both incident and reflected wavefields. Therefore, an improved method for updating subsurface parameters such as velocity, impedance and density to obtain an accurate Earth model that can account for both pressure and shear wavefields from seismic data is desired.